1. Field of the Invention
The present invention relates to a mobile communications system and mobile stations therefor. More particularly, the invention relates to a mobile communications system with mobility management functions, including location updating operations, which permit mobile switching centers to keep track of mobile stations roaming between different cells. The invention also to the structure of a mobile station for use in the system.
2. Description of the Related Art
The major components in a typical mobile communications system include: (a) mobile stations, or portable handsets carried by mobile users; (b) a plurality of base stations which support their respective radio service coverage areas, or cells; and (c) a mobile switching center which controls these base stations. When a mobile station places a call to another mobile station, this call request reaches the mobile switching center via a base station nearest to the calling mobile station. The mobile switching center, which tracks the location of every mobile station, sends a paging request message to another base station that is currently serving the called station. The base station transmits this message toward the called mobile station. If a paging response is returned, the system establishes a traffic channel between the calling station and called station, making it possible to transport voice or data signals.
Tasks of keeping track of the location of each mobile station are known as xe2x80x9cmobility management,xe2x80x9d which typically treats several adjacent cells as one unit for management purposes. The areas consolidated as such are called xe2x80x9clocation areas.xe2x80x9d FIG. 14 explains the concept of this location area, where smaller circles represent individual cells served by base stations named BS1 to BS40. Larger circles each including seven smaller circles denote location areas defined in this system.
Base stations are designed to transmit the identifier of the location area they belong to, thus allowing mobile stations within the area to recognize which location area they are currently visiting. The mobile stations update their locations by sending a location registration (or location update) message to the mobile switching center, when they move between different location areas. The mobile switching center records this information in its local and remote databases known as the home location register (HLR) and visitor location register (VLR). When there is an incoming call addressed to a specific mobile station, the mobile switching center first identifies the called station""s current location area, and then directs all the base stations in that area to transmit a paging request message.
Determining the size of a location area is not a simple task, because there are two conflicting factors that should be balanced. They are: paging message traffic and location update message traffic. If cells are grouped into smaller location areas, the traffic of paging request messages will decrease, because the number of base stations that simultaneously send a paging request message is reduced. However, this also means that mobile stations have to cross the border between location areas more often, thus sending an increased number of location update messages in total. On the other hand, larger location areas will reduce the total number of location updating messages, but instead, increase the paging messages traffic. Therefore, the system designer has to make a compromise between paging message traffic and location update message traffic, so as to reach the optimal location area size.
Another issue concerning location area design is that mobile users have their own activity patterns. They visit different places at different frequencies. For this reason, a specific setup optimized for one user may not always work effectively with another user. Researchers in this technical field have proposed various solutions for this problem as explained below.
One example is the Japanese Patent Application Laid-open Publication No. 8-84364 (1996), regarding a paging system for cellular telephone networks. This patent application proposes a special process to reduce the location updating traffic in such a network where a mobile station frequently moves between two location areas MLA1 and MLA2 shown in FIG. 15. First, the logs of outgoing and incoming calls are used to figure out which cells this mobile station is likely to visit, thereby identifying several frequently-visited cells. Then, the base stations serving those cells are grouped into what is called a xe2x80x9cvirtual mobile location area (VMLA).xe2x80x9d Referring to the example of FIG. 15, the VLMA involves three base stations BS12, BS16, and BS34. The mobile station registers this VLMA as its new location area.
Because the registered location area contains the cells the mobile station often visit, the necessity of location updating will be effectively reduced. About paging operations, the proposed system regards the base stations belonging to VMLA as the primary paging group, the rest of MLA1 as the secondary paging group, and the rest of MLA2 as the tertiary paging group. When a call is placed, the paging operation is attempted first in the primary paging group, then in the secondary group, and lastly in the tertiary group. In this way, the proposed system starts to page the mobile station from its VMLA, which is supposed to be the most likely place. As long as the number of base stations in the VMLA is less than in other areas such as MLA1 and MLA2, the proposed paging system will effectively reduce the amount of paging message traffic.
The above paging system, however, would not work as expected, when the mobile station tends to roam through a wide geographical area, or when each location area has to cover a relatively large number of cells as a result of introduction of microcells. In such situations, a larger VMLA would cause more paging messages to be transmitted. However, this is not to say that the system would work with a smaller VMLA, because the mobile station is less likely to respond to the first attempt of paging in a small area. When the intended mobile station is visiting outside the VMLA, the system is unable to connect with it immediately. This is because the paging process is programmed to start from the VMLA, and thus it takes some time to route the paging request to the right area.
Another solution is proposed in the Japanese Patent Application Laid-open Publication No. 9-116952 (1997), disclosing a mobile communications system. FIG. 16 is a diagram showing this proposed system, where the hatched circles represent the cells visited by a specific subscriber A, and the bold circles show those of another subscriber B. Location areas are defined on the basis of such activity pattern data of each subscriber, and mobile stations in this system are designed not to update their location register unless they leave their respective location areas, thus reducing the location update message traffic.
To describe the above mechanism more specifically, each mobile station measures how long it has been staying in a specific cell, and notifies the mobile switching center of the duration. If the notified duration is longer than a predetermined threshold, the mobile switching center registers the serving base station as a member base station of the location area being developed. When an incoming call to that mobile station is placed, the mobile switching center pages the station through all the member base stations within the location area. As such, the proposed system defines location areas according to each mobile station""s mobility pattern to avoid any possible increase in the location updating traffic.
Some active subscribers may visit too many cells to fit in a single location area. If this is the case, the mobile switching center evaluates the duration of their stay in each cell and sorts the relevant cells in descending order of the likelihood of their presence. In the example of FIG. 16, the subscriber A""s presence is classified into three levels (and represented by three different symbols) as follows: high level (small black circles), medium level (double circles), and low level (white circles). According to those levels, the cells are grouped into larger areas for paging purposes. When a call to this mobile station is detected, the mobile switching center first directs the high-level group of base stations to page the station. If no paging response is obtained there, it then commands the medium-level group to do the same. If it is still unable to hear the response, the paging request is routed to the low-level group. In this way, the proposed system offers a wide paging area, while taking advantage of the high likelihood of mobile presence. This makes it possible to suppress the paging message traffic, even when the mobile station tends to roam through a wide geographical area.
Mobile stations store all base station IDs relevant to their respective location areas. Suppose that one mobile station is leaving its location area. The mobile station will soon detect the departure by itself, because the base station ID received from the nearest base station does not coincide with any of the base station IDs they have. The mobile station now attempts to register its new location by using an ordinary system of location areas (i.e., those shown in FIG. 14), in order to keep on operating properly.
The above-described mobile communications system works fine as far as the activity of each mobile station is limited within a relatively small geographical region. However, this may not apply to such a subscriber who commutes to his/her distant office. In this case, two isolated location areas will be produced: one around his/her home, and the other around the office. Now, assume that there arises a call to his/her mobile station located in one location area with a lower probability of presence. Paging is attempted, however, in the other location area which is supposed to have a higher probability of presence. This attempt should end up with no response, only wasting time and bandwidth.
Again, the system is designed to combine most likely cells where a specific subscriber is expected to be found at a higher probability. However, it may not always be true that such cells are located closely to each other. With isolated location areas, location registration operations may not be performed efficiently.
Taking the above into consideration, an object of the present invention is to provide a mobile communications system which effectively suppresses the control message traffic when updating location registers and paging a specific mobile station.
To accomplish the above object, according to the present invention, there is provided a mobile communications system where base stations provide radio links for mobile stations in each cell and a mobile switching center controls connections between the mobile stations by switching circuits. In this system, each mobile station comprises the following element:
(1a) a notification message receiver which receives a notification message from the nearest base station;
(1b) a base station ID extractor which extracts a base station ID from the received notification message to identify which base station is sending the notification message;
(1c) a location change counting unit which detects a change in the mobile location by comparing two consecutive instances of the base station ID extracted by the base station ID extractor, and counts the detected location changes separately for each pair of base station IDs involved in each change;
(1d) a personal location area (PLA) setting unit which defines personal location areas by grouping associated cells into one area, based on the location change count values obtained by the location change counting unit;
(1e) a PLA memory which stores records of the personal location areas defined by the PLA setting unit, each of the records containing the base station IDs indicating which cells constitute each personal location area;
(1f) a location change detector which tests whether the personal location area registered at present is valid, each time the base station ID extracting unit extracts a base station ID that is different from before; and
(1g) a location registration unit which is activated when the location change detector has detected that the registered personal location area is no longer valid. It searches the PLA memory to retrieve one of the records of personal location areas that contains the extracted base station ID, and performs location registration by sending to the mobile switching center the retrieved personal location area and its relevant base station IDs.
Further, according to the proposed system, the mobile switching center comprises the following elements:
(2a) a PLA configuration data memory which stores records of the personal location area and relevant base station IDs received from each mobile station; and
(2b) a paging unit which searches the PLA configuration data memory, in response to an incoming call to one of the mobile stations, to retrieve one of the records that is relevant to the called mobile station, and pages the called mobile station through the base stations specified in the retrieved record.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.